powerpc/powernv: Merge common platform device initialisation

opal_ipmi_init and opal_flash_init are equivalent, except for the
compatbile string. Merge these two into a common opal_pdev_init
function.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>